1. Field of the invention
The present invention relates to a compound semiconductor field effect transistor, and more specifically to such a field effect transistor utilizing a semiconductor heterojunction.
2. Description of related art
In.sub.x Ga.sub.1-x As (x=0.530 which is a compound semiconductor having a lattice constant in match with that of an InP substrate, has an electron mobility and a saturation speed which are higher than those of GaAs, and therefore, can be said to be a semiconductor material more suitable to a field effect transistor (called "FET") operating at a super high frequency band of not less than 1 GHz. Therefore, various FET structures using this In.sub.x Ga.sub.1-x As (x=0.53) have been proposed.
However, since a barrier height in a Schottky junction between a metal and In.sub.x Ga.sub.1-x As (x=0.53) is as very small as 0.2 eV, when a metal Schottky junction gate FET (called "MESFET"), which has already reduced to practice in the case of GaAs, is constituted of In.sub.x Ga.sub.1-x As (x=0.53), a gate leakage current is too large. Because of this, the MESFET constituted of In.sub.x Ga.sub.1-x As (x=0.53) has not yet been reduced to practice. On the other hand, if a metal/oxide/semiconductor FET (called "MOSFET"), which is widely used in the field of silicon, is consitituted of In.sub.x Ga.sub.1-x As (x=0.53), there exists no suitable oxide insulator material which can form a good junction between In.sub.x Ga.sub.1-x As (x=0.53) and the oxide insulator material. Because of this, the MOSFET constituted of In.sub.x Ga.sub.1-x As (x=0.53) has not yet been reduced into practice.
On the other hand, In.sub.y Al.sub.1-y As (y=0.52) which is also one of compound semiconductors, has the lattice constant in match with that of In.sub.x Ga.sub.1-x As (x=0.53), and can be epitaxially grown on InP or In.sub.x Ga.sub.1-x As (x=0.53). For this feature, In.sub.y Al.sub.1-y As (y=0.52) including no impurity has possibility of being used as a gate insulator layer in a metal/insulator/semiconductor FET (called "MISFET") using InGaAs. For example, this type of MISFET has been proposed in IEEE Electron Device Letter, EDL-1,154(1980) and IEEE Electron Device Letter, EDL-1,174(1980). The proposed MISFET has a transconductance of 135 mS/mm in the case having a gate length of 0.6 .mu.m. This is superior to a GaAs MESFET having a similar device structure.
However, in the case of the mixture ratio y=0.52 of In.sub.y Al.sub.1-y As, the Schottky barrier height of the Schottky junction between the gate electrode and In.sub.y Al.sub.1-y As is only about 0.8 eV. In particular, when this MISFET is operated in an enhancement mode, since a positive gate bias voltage is applied, a gate leakage current reaches to about 10 A/cm.sup.-2 or more.
In order to overcome this problem, it may be considered to use, in place of InGaAs, Al.sub.z Ga.sub.1-z As (0&lt;z.ltoreq.1) which has the Schottky barrier height of greater than 0.8 eV. In this case, since AlGaAs and InGaAs are different in lattice constant, a lattice match cannot be realized, and therefore, dislocation defects of crystal occur at a heterojunction boundary between AlGaAs and InGaAs because of difference in lattice constant. As a result, a new problem such as unstable characteristics of FET occurs.